This invention relates to electromagnetic tagging apparatus and in particular to magnetic tags for use with electromagnetic identification systems of the kind which can be employed, for example, in electronic article surveillance (E.A.S.), primarily for in-store sucurity, or in access control.
Such systems, in either environment, utilise an element, frequently referred to a "tag", which influences an electromagnetic field, so causing a disturbance in a characteristic of the field as detected by a detector device. The tag needs to exhibit a number of characteristics one of which, especially relevant for in-store usage, is the capability of being readily de-activated by check-out personnel whilst being difficult for a would-be thief to de-activate.
It is usual for the element, or tag, to contain a strip of highly permeable magnetic material. Such material is easily influenced by a relatively weak interrogating field and caused to generate a number of harmonic frequencies which are readily detected by a suitable detection device. Unless it is proposed to physically fracture or mechanically strain the strip in order to de-activate it, and such activities would place severe limitations upon the form which a tage could take, de-activation is usually achieved by magnetising hard material disposed in the tag and configured so as to produce, when so magnetised, a series of poles along the length of the strip of highly permeable material. This series of lpoles alters the magnetic "profile" of the tag, as presented to the interrogating field, and inhibits th generation of the aforementioned harmonics, thus permitting the detection device to distinguish between activated and de-activated tags.
It has been usual hitherto for the magnetically hard material used for de-activation of the tag to be provided either in the form of discontinuous lengths arranged in close proximity to the strip of magnetically soft material, or in continuous lengths similarly disposed. In the first case, de-activation is relatively straightforward for check-out personnel to accomplish, but the same can be said for would-be thieves. In the second case, more care has to be taken by the check-out personnel because the continuous length of magnetically hard material has to be selectively magnetised to produce a pole pattern sufficient to change the electromagnetic profile of the tag, but at the same time de-activation is made more difficult for the would-be thief.